1. Field of the Invention
The present invention relates, generally to a control scheme for cooling a dual clutch transmission as used in motor vehicle driveline and, more specifically, to a method of controlling the flow of cooling fluid provided to each of the two clutches of a dual clutch transmission.
2. Description of the Related Art
Generally speaking, land vehicles require a powertrain consisting of three basic components. These components include a power plant (such as an internal combustion engine), a power transmission, and wheels. The power transmission component is typically referred to simply as the xe2x80x9ctransmission.xe2x80x9d Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle. Presently, there are two typical transmissions widely available for use in conventional motor vehicles. The first, and oldest type is the manually operated transmission. These transmissions include a foot operated start-up or launch clutch to engage and disengage the driveline with the power plant and a gearshift lever to selectively change the gear ratios within the transmission. When driving a vehicle having a manual transmission, the driver must coordinate the operation of the clutch pedal, the gearshift lever and the accelerator pedal to achieve a smooth and efficient shift from one gear to the next. The structure of a manual transmission is simple and robust and provides good fuel economy by having a direct power connection from the engine to the final drive wheels of the vehicle. Additionally, since the operator is given complete control over the timing of the shifts, the operator is able to dynamically adjust the shifting process so that the vehicle can be driven most efficiently. The disadvantages of the manual transmission is that there is an interruption in the drive connection during gear shifting and that there is a great deal of required physical interaction on the part of the operator to shift gears.
The second, and newer choice for the transmission of power in a conventional motor vehicle is an automatic transmission. First and foremost, automatic transmissions offer ease of operation. The driver of a vehicle having an automatic transmission is not required to use both hands, one for the steering wheel and one for the gearshift, and both feet, one for the clutch and one for the accelerator and brake pedal in order to safely operate the vehicle. In addition, an automatic transmission provides greater convenience in stop and go situations, because the driver is not concerned about continuously shifting gears to adjust to the ever-changing speed of traffic. Although conventional automatic transmissions avoid an interruption in the drive connection during gear shifting, they suffer from the disadvantage of reduced efficiency because of the need for hydrokinetic devices, such as torque converters, interposed between the output of the engine and the input of the transmission for transferring kinetic energy therebetween.
More specifically, torque converters typically include impeller assemblies that are operatively connected for rotation with the torque input from an internal combustion engine, a turbine assembly that is fluidly connected in driven relationship with the impeller assembly and a stator or reactor assembly. These assemblies together form a substantially toroidal flow passage for kinetic fluid in the torque converter. Each assembly includes a plurality of blades or vanes that act to convert mechanical energy to hydrokinetic energy and back to mechanical energy. The stator assembly of a conventional torque converter is locked against rotation in one direction but is free to spin about an axis in the direction of rotation of the impeller assembly and turbine assembly. When the stator assembly is locked against rotation, the torque is multiplied by the torque converter. During torque multiplication, the output torque is greater than the input torque for the torque converter. However, when there is no torque multiplication, the torque converter becomes a fluid coupling. Fluid couplings have inherent slip. Torque converter slip exists when the speed ratio is less than 1.0 (RPM input greater than than RPM output of the torque converter). The inherent slip reduces the efficiency of the torque converter.
While torque converters provide a smooth coupling between the engine and the transmission, the slippage of the torque converter results in a parasitic loss, thereby decreasing the efficiency of the entire powertrain. Further, the torque converter itself requires pressurized hydraulic fluid in addition to any pressurized fluid requirements for the actuation of the gear shifting operations. This means that an automatic transmission must have a large capacity pump to provide the necessary hydraulic pressure for both converter engagement and shift changes. The power required to drive the pump and pressurize the fluid introduces additional parasitic losses of efficiency in the automatic transmission.
In an ongoing attempt to provide a vehicle transmission that has the advantages of both types of transmissions with fewer of the drawbacks, combinations of the traditional xe2x80x9cmanualxe2x80x9d and xe2x80x9cautomaticxe2x80x9d transmissions have evolved. Most recently, xe2x80x9cautomatedxe2x80x9d variants of conventional manual transmissions have been developed which shift automatically without any input from the vehicle operator. Such automated manual transmissions typically include a plurality of power-operated actuators that are controlled by a transmission controller or some type of electronic control unit (ECU) to automatically shift synchronized clutches that control the engagement of meshed gear wheels traditionally found in manual transmissions. The design variants have included either electrically or hydraulically powered actuators to affect the gear changes. However, even with the inherent improvements of these newer automated transmissions, they still have the disadvantage of a power interruption in the drive connection between the input shaft and the output shaft during sequential gear shifting. Power interrupted shifting results in a harsh shift feel which is generally considered to be unacceptable when compared to smooth shift feel associated with most conventional automatic transmissions.
To overcome this problem, other automated manual type transmissions have been developed which can be power-shifted to permit gearshifts to be made under load. Examples of such power-shifted automated manual transmissions are shown in U.S. Pat. No. 5,711,409 issued on Jan. 27, 1998 to Murata for a Twin-Clutch Type Transmission, and U.S. Pat. No. 5,966,989 issued on Apr. 4, 2000 to Reed, Jr. et al for an Electro-mechanical Automatic Transmission having Dual Input Shafts. These particular variant types of automated manual transmissions have two clutches and are generally referred to simply as dual, or twin, clutch transmissions. The dual clutch structure is most often coaxially and cooperatively configured so as to derive power input from a singular engine flywheel arrangement. However, some designs have a dual clutch assembly that is coaxial but with the clutches located on opposite sides of the transmissions body and having different input sources. Regardless, the layout is the equivalent of having two transmissions in one housing, namely one power transmission assembly on each of two input shafts concomitantly driving one output shaft. Each transmission can be shifted and clutched independently. In this manner, uninterrupted power upshifting and downshifling between gears, along with the high mechanical efficiency of a manual transmission is available in an automatic transmission form. Thus, significant increases in fuel economy and vehicle performance may be achieved through the effective use of certain automated manual transmissions.
The dual clutch transmission structure may include two disc clutches each with their own clutch actuator to control the engagement and disengagement of the two-clutch discs independently. While the clutch actuators may be of the electromechanical type, since a lubrication system within the transmission is still a necessity requiring a pump, some dual clutch transmissions utilize hydraulic shifting and clutch control. These pumps are most often gerotor types, and are much smaller than those used in conventional automatic transmissions because they typically do not have to supply a torque converter. Thus, any parasitic losses are kept small. Shifts are accomplished by engaging the desired gear prior to a shift event and subsequently engaging the corresponding clutch. With two clutches and two inputs shafts, at certain times, the dual clutch transmission may be in two different gear ratios at once, but only one clutch will be engaged and transmitting power at any given moment. To shift to the next higher gear, first the desired gears on the input shaft of the non-driven clutch assembly are engaged, then the driven clutch is released and the non-driven clutch is engaged.
This requires that the dual clutch transmission be configured to have the forward gear ratios alternatingly arranged on their respective input shafts. In other words, to perform upshifts from first to second gear, the first and second gears must be on different input shafts. Therefore, the odd gears will be associated with one input shaft and the even gears will be associated with the other input shaft. In view of this convention, the input shafts are generally referred to as the odd and even shafts. Typically, the input shafts transfer the applied torque to a single counter shaft, which includes mating gears to the input shaft gears. The mating gears of the counter shaft are in constant mesh with the gears on the input shafts. The counter shaft also includes an output gear that is meshingly engaged to a gear on the output shaft. Thus, the input torque from the engine is transferred from one of the clutches to an input shaft, through a gear set to the counter shaft and from the counter shaft to the output shaft.
Gear engagement in a dual clutch transmission is similar to that in a conventional manual transmission. One of the gears in each of the gear sets is disposed on its respective shaft in such a manner so that it can freewheel about the shaft. A synchronizer is also disposed on the shaft next to the freewheeling gear so that the synchronizer can selectively engage the gear to the shaft. To automate the transmission, the mechanical selection of each of the gear sets is typically performed by some type of actuator that moves the synchronizers. A reverse gear set includes a gear on one of the input shafts, a gear on the counter shaft, and an intermediate gear mounted on a separate counter shaft meshingly disposed between the two so that reverse movement of the output shaft may be achieved.
While these power-shift dual clutch transmissions have overcome several drawbacks associated with conventional transmissions and the newer automated manual transmissions, it has been found that controlling and regulating the automatically actuated dual clutch transmission to achieve the desired vehicle occupant comfort goals is a complicated matter. There are a large number of events to properly time and execute within the transmission for each shift to occur smoothly and efficiently. In addition, the clutch and complex gear mechanisms, working within the close confines of the dual clutch transmission case, generate a considerable amount of heat. The heat build-up is aggravated by the nature of the clutch mechanisms themselves, each of which are typically constructed of two series of plates, or discs, one set connected in some manner to the output of the engine and the second attached to an input shaft of the transmission. Each of the set of plates may be covered or impregnated by a friction material. The clutch plates and discs are pressed together under pressure to a point at which the plates and discs make a direct physical connection. The clutch may be designed for a full xe2x80x9clockupxe2x80x9d of the plates and discs, or may be designed with a certain amount of xe2x80x9climited slipxe2x80x9d. Regardless, the slipping of the friction plates within a friction type clutch, whether from a designed limited slip or the normal uncontrolled slipping that occurs during clutch engagement and disengagement, generates heat that needs to be dissipated. A considerable amount of heat can be generated in the typical dual clutch transmission utilizing a combined coaxial clutch assembly wherein the one clutch fits within the second clutch.
In order to provide sufficient cooling to the clutch assemblies of the conventional dual clutch transmission, the clutch assemblies are usually bathed in transmission fluid in a generally uncontrolled manner. While this approach has generally worked for its intended purpose, disadvantages still remain. Specifically, conventional clutch cooling control schemes or methods have either failed to adequately provide for proper cooling and heat reduction of the clutches of the dual clutch transmission or have resulted in producing large efficiency losses by excessively flooding of the clutch assemblies with fluid. Inadequate cooling strategies have led to shortened component life due to damage and ultimate failure of the clutch assemblies within the dual clutch transmission. Similarly, inadequate cooling is responsible for rapid breakdown of the physical properties of the transmission fluid, which can cause failure of the other components within the transmission. The conventional methods of employing a fluid bath or providing a direct uncontrolled flow of cooling fluid to the clutch assembly also cause unnecessary clutch drag and put excessive demands on the pump resulting in poor clutch life and lower fuel efficiencies.
Accordingly, there exists a need in the related art for an improved method of controlling the cooling of the clutch assemblies of the dual clutch transmissions. Specifically, there is a need to provide a method of controlling the delivery and flow of cooling fluid to the clutches in a dual clutch transmission where the structure of the transmission provides for the separate routing and regulation of the transmission fluid to each clutch for independent clutch cooling. In this manner, the uncontrolled and constant fluid flow of conventional cooling approaches, which are excessive and inefficient are avoided, while, at the same time, optimum cooling flow is provided. Such a method properly cools the clutches to prevent component damage, reduces unnecessary clutch drag and reduces excessive demands on the pump, thereby providing for long clutch life and increased fuel economy.
The disadvantages of the related art are overcome by the method of the present invention for controlling the fluid cooling of the clutches of a dual clutch transmission disposed within a vehicle. More specifically, the method of the present invention controls the temperature of the clutches of a dual clutch transmission and includes the steps of providing a predetermined flow of cooling fluid to at least one clutch of the dual clutch transmission for controlling bulk clutch temperature, monitoring the temperature of the cooling fluid at the clutch, and changing the flow of the cooling fluid to the clutch as a function of a change in the bulk clutch temperature. The method of the present invention has the advantage of providing only the necessary amount of cooling fluid to the clutches without excessively flooding them with cooling fluid, which is inefficient and causes drag and introduces parasitic losses to the components of the dual clutch transmission.
Another advantage of the present invention is provided by the accurate determination of temperature changes to the clutches by monitoring the power transfer across the clutches rather than constantly taking empirical temperature measurements. Thus, another embodiment of the method of the present invention includes the steps of monitoring the input torque applied to the clutches and monitoring the slip across the clutches. The method also determines the change in power transferred across the clutches when either the input torque or slip values changes as well as the change in the bulk clutch temperature of each clutch. In this way, the required change in the flow of cooling fluid to account for the change in the bulk clutch temperatures of each clutch may be determined. The method then determines the available cooling fluid flow from the pump for the current engine speed and proportions the available cooling fluid flow to each of the clutches to account for the change in bulk clutch temperature of each clutch. Thus, each clutch is apportioned the proper amount of the available cooling fluid flow based on its power transfer.
Another advantage is provided by accounting for changes in the bulk clutch temperatures due to the cooling fluid flow provided to the clutches during the prior pass through the method steps of the present invention. Thus, the method of the present invention determines the power transferred across the clutches and determines an initial change in the bulk clutch temperature for each clutch based on the power transferred across each clutch. Then, the method determines a secondary change in the bulk clutch temperatures based on the existing cooling fluid flow to each clutch during the prior pass through the method steps and uses this value to determine a total change in bulk clutch temperature for each clutch by summing the initial change with the secondary change.
Another advantage of the present invention is provided by accounting for changes to the temperature of the cooling fluid itself. By determining the difference between the temperature of the cooling fluid in the sump and the temperature of the cooling fluid leaving the clutches, the method determines an additional required change in the flow of cooling fluid for each clutch based on this temperature difference. The method then uses this temperature difference to determine the total required change in the flow of cooling fluid for each clutch.
In this manner, the method of the present invention overcomes the limitations and drawbacks of the prior art by controlling the cooling of the clutches of a dual clutch transmission and accounting for the changes in the bulk clutch temperatures due to the cooling fluid flow provided to each clutch prior to any change in power transferred across the clutches and by accounting for increases in the temperature of the cooling fluid as the clutches power shift.